1. Field of the Invention
This invention relates to a rod lens array and also to a printer head and an optical printer using such a rod lens array. A rod lens array according to the invention is adapted to cause a plurality of light emitting elements to emit light and form highly accurate and sharp images of the elements on the surface of a photosensitive member that operates as recording medium. A rod lens array according to the invention can suitably be used in a printer head or an optical printer comprising LCDs (liquid crystal display elements) or LEDs (light emitting diodes) to name a few.
2. Related Background Art
Known graded refractive index rod lens arrays (hereinafter referred to simply as xe2x80x9crod lens arrayxe2x80x9d) used in optical printers such as liquid crystal printers and LED printers typically have a configuration as shown in FIG. 1 of the accompanying drawings, which is a schematic cross sectional view of a known rod lens array taken along a direction perpendicular to the optical axes of the rod lenses. Referring to FIG. 1, a plurality of rod lenses 31 are densely arranged and aligned relative to each other by means of a pair of side panels 32 to form a rod lens array 30. Subsequently, the gaps separating the rod lenses 31 are filled with opaque resin 33 to securely hold the rod lenses in position. In this way, the rod lens array 30 is prepared as an oblong image forming device for forming an erecting life-size image.
Normally, the side panels 32 are made of fiber-reinforced plastic or FRP. Therefore, the surfaces of the lateral walls 32 generally show undulations of a dimension of 6 to 8 xcexcm reflecting the fibers contained therein.
FIG. 2 is a schematic cross sectional view of a principal portion of an optical printer realized by using such a rod lens array, illustrating the image forming effect of the rod lens array in an ideal state. In FIG. 2, the components same as or similar to those in FIG. 1 are denoted respectively by the same reference symbols and would not be described any further.
Referring to FIG. 2, light emitting means (an array of light emitting elements) 45 comprises a plurality of light emitting elements that are adapted to emit respective light beams according to the image signal applied thereto. Then, the light beams are focussed to an imaging spot 44 on the surface of a photosensitive drum 46 operating as recording medium to form an erecting life-size image by means of a rod lens array (imaging device) 30 comprising a plurality of rod lenses 31 arranged in array. Thus, the light emitting means 45 and the photosensitive drum 46 are arranged vis-a-vis with the rod lens array interposed therebetween. With this arrangement, a latent image of an object can be formed on the surface of the photosensitive drum of the optical printer. Note that the optical printer head of the optical printer includes the light emitting element array 45 and the rod lens array 30.
Known optical printer heads and optical printers comprising such a light emitting element array mostly provide a resolution of about 300 dpi. However, in response to the increased demand for high definition printer in recent years, optical printer heads and optical printers providing a high resolution of 600 dpi or 1,200 dpi are currently being marketed. Rod lens arrays used in such optical printer heads and optical printers are therefore required to meet rigorous imaging requirements that have not been experienced.
In other words, the image (of the light emitting elements) formed at the specified spot on the surface of the photosensitive drum is required to be dimensionally reduced in the case of a high definition optical printer so that each of the rod lenses has to focus the light beam passing therethrough precisely at the specified spot.
If the rod lenses are allowed to produce an ideal image in a condition where they are completely free from perversive factors, it is possible for the plurality of rod lenses 31 to form an image of a single spot on the surface of the photosensitive drum 46 as shown in FIG. 2 if the rod lenses 31 are arranged with little positional variances as shown in FIG. 1.
However, if a perversive factor is involved in any of the rod lenses of the array as shown in FIG. 3, the light beam emitted from the rod lens 51 involving a perversive factor is not focussed at the right spot to give rise to a phenomenon referred to as stray dot. Thus, FIG. 3 is a schematic cross sectional view of a principal portion of an optical printer realized by using a rod lens array, illustrating the image forming effect of the rod lens array when some of the rod lenses 51 involve a perversive factor. In FIG. 3, reference symbol 50 denotes a rod lens array and reference symbol 52 denotes a side panel, while reference symbols 55 and 54 denote a light emitting element array and a spot formed by a focussed light beam respectively.
Since known dot lenses have a large diameter and a sufficient strength while they are not required to show a high resolution, the stray dots formed by them, if ever, do not significantly affect the overall effect of the dot lens array.
However, in the case of an optical printer head or an optical printer required to show a resolution of 600 dpi or more, any cyclical changes in the formation of dots can result in an uneven density distribution of the obtained image that normally involves half tones to give rise to a serious problem.
One of the causes of the perversive factors of the rod lenses of a rod lens array is the undulations of a dimension of 6 to 8 xcexcm due to fibers contained in the fiber-reinforced plastic (FRP) of the side panels. An uneven density distribution is produced in the obtained image when the spot formed by a rod lens is displaced from the right position by more than ⅕ of the size of a pixel required for the specified resolution of the printer.
In view of the above circumstances, it is therefore the object of the present invention to provide a rod lens array that can suppress any perversive factors of each of the rod lenses to eliminate the possible displacement of the spot formed by the rod lens and a printer head and an optical printer comprising such a rod lens array.
According to the invention, the above object is achieved by providing a rod lens array to be used for an optical printer, the rod lens array comprising:
a plurality of rod lenses arranged densely in array;
an opaque substance filled in the gaps separating the plurality of rod lenses; and
a pair of side panels sandwiching the plurality of rod lenses and the opaque substance;
the rod lens array satisfying the requirement as defined by formula below;
S less than (Z/L)xc2x7(25.4/5P), 
P being the resolution of the optical printer expressed in dpi, Z being the length of the rod lenses expressed in mm, L being the conjugate length of the rod lenses expressed in mm, S being the flatness of the surfaces of the side panels bearing the rod lenses.
In another aspect of the invention, there is provided a printer head to be used for an optical printer, the printer head comprising:
a light emitting element array formed by arranging a plurality of light emitting elements;
a plurality of rod lenses arranged densely in array to focus the light beams emitted respectively from the light emitting elements of the light emitting element array to a position separated by a predetermined distance;
an opaque substance filled in the gaps separating the plurality of rod lenses; and
a pair of side panels sandwiching the plurality of rod lenses and the opaque substance;
the rod lens array satisfying the requirement as defined by formula below;
S less than (Z/L)xc2x7(25.4/5P), 
P being the resolution of the optical printer expressed in dpi, Z being the length of the rod lenses expressed in mm, L being the conjugate length of the rod lenses expressed in mm, S being the flatness of the surfaces of the side panels bearing the rod lenses.
In still another aspect of the invention, there is provided an optical printer comprising:
a light emitting element array formed by arranging a plurality of light emitting elements;
a photosensitive member arranged at a position separated from the light emitting element array by a predetermined distance;
a plurality of rod lenses arranged densely in array to focus the light beams emitted respectively from the light emitting elements of the light emitting element array on the photosensitive member;
an opaque substance filled in the gaps separating the plurality of rod lenses; and
a pair of side panels sandwiching the plurality of rod lenses and the opaque substance;
a developing unit for developing an electrostatic latent image formed on the photosensitive member by irradiating the photosensitive member with the light beams emitted from the light emitting elements to a toner image;
a transfer unit for transferring the developed toner image onto an image receiving member;
a fixing unit for fixing the transferred toner image on the image receiving member;
the rod lens array satisfying the requirement as defined by formula below;
S less than (Z/L)xc2x7(25.4/5P), 
P being the resolution of the optical printer expressed in dpi, Z being the length of the rod lenses expressed in mm, L being the conjugate length of the rod lenses expressed in mm, S being the flatness of the surfaces of the side panels bearing the rod lenses.